This invention relates generally to the use of ultrasonics in medical technology applications, and more particularly to a method and apparatus for applying high intensity focused ultrasound through nonhomogeneous volumes.
Sound waves that have a frequency greater than approximately 20 kHz are referred to as ultrasound. Much of the research and development in the field of medical ultrasound relates to diagnostic applications and therapeutic applications. Medical diagnostic ultrasound systems are useful for generating images of anatomical structures within a patient's body. The images are obtained by scanning a target area with waves of ultrasound pulses. In therapeutic ultrasound applications, high intensity ultrasound pulses are transmitted into a target area to induce changes in state of the target. High intensity ultrasound pulses induce changes in state through thermal effects (e.g., induced hyperthermia) and mechanical effects (e.g., induced cavitation).
In medical ultrasound applications, ultrasound pulses are delivered to a patient by an ultrasound transducer. To obtain the ultrasound pulses, electronic signals are input to the transducer. The transducer then converts the electrical signals into ultrasound pulses which are transmitted into the patient's body as an ultrasound beam. Such ultrasound beam is absorbed, dispersed, and reflected. Diagnostic applications take advantage of the reflected ultrasound energy, which is analyzed and processed to generate image and flow information. Therapeutic applications take advantage of the absorbed ultrasound energy to change the state of a target area. Dispersion is an aspect of ultrasound technology that poses challenges to be overcome. Specifically in therapeutic applications, it is desired that ultrasound energy cause a change of state at the target area and not adversely impact other volumes within the patient. Refractive dispersion diminishes the therapeutic effect of ultrasound energy and causes the ultrasound energy to be absorbed at unintended areas. Accordingly there is a need for accurately focussing the HIFU beam.
In a hyperthermia HIFU treatment the ultrasound beam is highly focused. The beam intensities increase along the beam path from the transducer to the target area. At the target area very high temperatures can be induced. The absorption of the ultrasound energy at the target area induces a sudden temperature rise (e.g., tens of degrees centigrade per second). This temperature rise causes coagulation or ablation of target area cells. Accordingly, hyperthermia HIFU treatments can cause damage to an internal lesion. It is desirable that such damage occur without harming intermediary cells between the transducer and the target area.
HIFU treatments are desired for rapid heating of human tissues to arrest bleeding and to ablate tumors. Typically, a target area is located deep in the body and the ultrasound energy is delivered through the skin. Using an intensity in excess of 1000 W/cm.sup.2 spatial peak continuous wave (SPCW), the target area is heated at a rate of approximately 25.degree. C./second. To limit the rapid heating effects to the desired target area, and spare the intervening tissue, a large aperture transducer is used. Along the path to the target area, however, the ultrasound energy travels through fat and muscle, (i.e., the ultrasound travels through a nonhomogeneous medium). The ultrasound travels at different speeds through the different materials resulting in refractive dispersion of the ultrasound pulses. This causes difficulty in focusing to the desired target area. This invention is directed to improving focus of high intensity ultrasound through nonhomogeneous tissue so that (i) thermal effects in intermediary tissue are controlled and harmless, and (ii) an effective therapeutic dose is delivered to the target volume.
The intensities described in this application are specified as spatial peak continuous wave (SPCW), spatial peak temporal average (SPTA), or spatial peak temporal peak (SPTP). Referring to FIG. 1A, a SPCW waveform 21 is shown. The dotted portion 23 indicates negative pressures which are cut off. Conventional diagnostic ultrasound applications use ultrasound pulses with intensities of up to 100 mW/cm.sup.2 spatial peak temporal average (SPTA) as exemplified by waveform 25 of FIG. 1B. The dotted portion 27 indicates negative pressures which are cut off. Referring to FIG. 1C, a SPTA waveform 29 is shown. Each of these waveforms 21, 25, 29 are for a 1 MHz ultrasound waveform.